DK153223B - METHOD AND APPARATUS FOR MANUFACTURING SILICON RODS WITH uniform cross-section - Google Patents

Description

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DK 153223 B

The present invention relates to a method for producing silicon rods of uniform cross-section and of the kind specified in the preamble of claim 1, and to an apparatus for use in the practice of this method.

It is known to produce semi-conductive, ultra-pure silicon rods by pyrolysis or reduction with hydrogen of an gaseous silicon compound such as, for example, monosilane, silicon tetrachloride or trichlorosilane on a rod-shaped red-hot silicon or metal support body having a high melting point and good electronic conductivity number. Known methods and apparatus include disclosed in U.S. Patents 3,099,534, 3,011,877 and 3,147,141, according to which the gaseous silicon compound is introduced from the bottom and the top of the pyrolysis vessel, respectively. This causes the rods produced by the pyrolysis to have the greatest diameter at the bottom and the top of the container, respectively, and this tendency is reinforced by the increasing diameter of silicon growing. The same is true when the silicon-containing compound is supplied through several feed orifices distributed along the support bodies (DE-AS 12 92 640).

Before using the generated rods for suspension or flow zone fusion, they are ensured that they are of uniform diameter or approximately perfectly circular cylinder shape by scraping off excess material.

The object of the invention is to provide a process for the production of ultra-pure silicon rods by pyrolysis of monosilane in such a way that the rods are of uniform cross-section.

This is achieved according to the invention by the method according to claim 1 and by the apparatus according to claim 2, respectively.

Thus, according to the invention, the proportional proportion of monosilane introduced at various levels is regulated in the container. Naturally, at each level, the supply mouths in question produce uniform monosilane quantities. As the rod diameter increases because silicon is deposited by the pyrolysis,

DK 153223B

- 2 - this proportion increases with increasing level in the container. Hereby it can be achieved that the generated rods are given the same diameter over the entire length of the rod.

The invention will now be described in more detail with reference to the drawing, in which: 1 is a vertical section through an apparatus for use in the practice of the invention; FIG. 2 is a section along line II-II of FIG. 1 and 10 in FIG. 3 shows schematically the location in the device of supply mouths.

The apparatus shown in the drawing comprises a container 3 and a bottom 4. The container wall is hollow, so that it can be cooled by a cooling medium flowing through an inlet nozzle 11 and an outlet nozzle 12. At the top, the container 3 is provided with an outlet nozzle 7 for release of gases produced by thermal decomposition in the interior of the container. Furthermore, the container wall is provided with windows 21 through which the process which takes place in the interior of the container 20 can be followed.

Also the bottom 4 is formed with voids so that a cooling medium is fed through the bottom via an inlet and outlet nozzle 13 and 14 respectively.

Four copper electrodes 8 are fixed vertically above the bottom 4 by means of thermal insulators 9 symmetrically about the center thereof. Each electrode 8 is connected via a terminal 10 to a voltage source not shown, and the lower part of the electrode is flowed through a cooling medium via an inlet and outlet pipe 18 and 19. Via a tantalum disc 20, each electrode 8 is connected at the top with a vertical rod 1 of ultra pure silicon.

The silicon rods 1 are two and two at their upper end interconnected by an ultrasonic silicon rod 2. The rods 1 are furthermore two and two separated by a thermal insulator 5 of substantially cross-sectional cross-section. The insulator 5 is located centrally in the container and has an internal cavity flowing through a refrigerant

DK 153223B

- 3 - via inlet and outlet pipes 15 and 16. In the thermal insulator 5 there is also a feeder tube 17 for monosilane.

In three levels, from the feed tube 17, four branch pipes are provided, the outlets 6 of which are located in the four outer 5 points of the insulator 5 at that level.

A tube 22 is also passed through the bottom 4 through which a preheating hot air stream can be fed into the container.

As shown in FIG. 3, the feeder tube 17 is branched into an upper, an intermediate and a lower level 10. For each level there is a flow regulator C. With these regulators, the monosilane influx at each level and the proportional proportion of monosilane supplied at the upper level can be increased with the growing diameter of the silicon rods at the silicon deposition. By means of a flow divider S in each level, it is ensured that the supply orifices 6 at that level deliver uniform amounts of monosilane. Deviations should preferably be less than a few percent.

The invention will now be further illustrated by an example.

In an apparatus such as that shown in FIG. 1 and 2, the support bodies were heated to a temperature of 8-900 ° C and the amount of monosilane supplied was adjusted such that the ultra-silicon deposition rate on the support bodies was 4-8 µm. The following table shows results obtained under different conditions:

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Table

Sample Rod- Distribution of monosilane- Distribution of rod slide no. The diaphragm supply between meters meter upper middle bottom upper middle bottom (mm) stepped on produced rod level 5 (1) 50 1 00 1.10 1.05 1.00 (2) 50 0 0 1 1.00 1 , 20 1.30 (3) 100 1 0 0 1.30 1.10 1.00 (4) 50 1 11 1.00 1.03 1.07 (5) 100 1 1 1 1.00 1.08 1 , 10 10 (6) 30 1.03 1 0.97 1.00 1.00 1.00 (7) 50 1.1 1 0.9 1.02 1.00 1.01 (8) 70 1.2 1 0.8 1.02 1.00 1.02 In the table, samples Nos. 1-3 are prepared by the usual method with the addition of monosilane alone at the top or bottom of the container. Samples Nos. 4 and 5 are prepared by applying equal amounts of monosilane at the three levels, while Nos. 6-9 are prepared by the method of the present invention.

It is clear from the table that the invention provides for the manufacture of silicon rods whose diameter variation in the length of the rods is quite small. The rods can be used immediately in a zone melting process.

It should be further noted that in the process of the invention, amorphous silicon contaminants do not occur due to homogeneous reactions in the gas phase.

In the following, the use of the apparatus described will be explained.

When the ultra-clean silicon rods 1, which serve as 30 supporting bodies, are placed on the copper electrodes 8 or rather on the tantalum discs 20 and connected above each other by the transverse rods 2, the container 3 is placed over the bottom 4 and the outlet socket 7 is connected to a valve in a suction assembly not shown which can evacuate the container. The air is then extracted into the container until a predetermined negative pressure is obtained in the container. then

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- 5 - the suction unit valve is closed and a hot air stream is blown into the container through the tube 22 so that the silicon rods 1 are heated. When a positive pressure is built up in the container 3, the valve is opened again and the rods 1 are heated, 5 until the predetermined temperature is reached. At the same time, refrigerant is passed through the container wall, the bottom and the thermal insulator.

When the silicon rods 1 of the air stream are brought up to the predetermined temperature, they are applied via the copper electrodes 8 to an electric current, so that the temperature of the rods 1 is raised by resistance heating above 800 ° C. Thereafter, the hot air flow is interrupted and monosilane is introduced through the pipe 17. The flow rate of the introduced monosilane is controlled by appropriate regulators and it is ensured that uniform quantities flow out through the four orifices 6 in each level. The monosilane introduced into the container is subjected to pyrolysis on the red-hot, ultra-pure silicon rods 1, thereby depositing ultra-pure silicon on these rods.

Due to the design of the thermal insulator 5, the ultra-clean silicon rods 1, although their diameter eventually grows with the deposition of additional material, will not be exposed to radiant heat from one or more of the other rods 1, and the rods are therefore not affected by the homogeneous rods. 25 reactions in the gas phase. The growing diameter of the rods therefore remains uniform in all sections through the rods.

Furthermore, since the homogeneous reaction in the gas can be controlled, increased productivity and yield of higher quality can be achieved.

In the following, the invention is illustrated in more detail by some examples.

Example 1

Four support bodies in the form of ultra-clean silicon rods, each with a diameter of 5 mm and a length of 1200 mm, were placed vertically and spaced apart and a common axis of symmetry in a pyrolysis vessel. The upper ends of the support members were connected two by two by means of ultra-clean silicon rods 5 mm in diameter and a - 6 -

DK 153223 B

length of 300 mm. The connecting rods had a resistance of 50 SI cm. A thermal insulator such as that shown in the drawing was placed between the support bodies. After preheating by means of a hot air stream, the support bodies are maintained at a temperature of about 850 ° C and the monosilane is passed into the container through the upper, middle and lower mouths 6. The monosilane is subjected to pyrolysis on the red-hot support bodies and the rate at which silicon deposited on the bodies is determined at 4-8 µm / min, as in the prior art processes of this kind, until the diameter of each silicon rod reaches 60 mm. The time taken for the rod diameter to reach 60 mm can be reduced by about 10% compared to known methods.

The polycrystalline silicon rods thus produced are subjected to a suspension zone fusion process and the monocrystallization of these rods was improved by about 30%.

Example 2 When ultra-pure silicon rods, each with a diameter of 20 mm, are produced at a pyrolysis temperature of about 900 ° C and a deposition rate of about 25% more than indicated in Example 1, the time spent in pyrolysis can be shortened by about 23%. The monocrystallization is essentially as described in Example 1.

Example 3

When prepared under the conditions of Example 1 of ultra-pure silicon rods with a diameter of 100 mm, the time taken for pyrolysis can be reduced by about 25%. As is known in the art, there is no contamination with powdered silicon formed by homogeneous reaction in the gas phase.

Example 4

Ultra-pure silicon was made in rods with a diameter of 60 mm, with twelve support members exposed to pyrolysis at the same time. The supporting bodies were divided into four groups by a thermal insulator. One of the groups - comprising three supporting bodies - was placed in separate compartments in pyrolysis.

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- 7 - container. It was found that by using a thermal insulator the pyrolysis time could be reduced by about 6%, while the product yield increased by about 4%.

Thus, according to the invention, a cooled thermal insulator is arranged between the red-hot supporting bodies so that no supporting body is exposed to radiant heat from another supporting body. The thermal insulator can be made of the same material as the container in which the pyrolysis takes place and cooled in the same way as it is by water or another refrigerant whose temperature can be controlled. Conveniently, the thermal insulator should have a sufficient height for the majority of the silicon support bodies to be shielded from each other and preferably positioned so that the pyrolysis vessel is divided into uniform compartments. Where the container contains a greater number of carriers, it may be sufficient that groups of carriers be thermally insulated from one another, that is, without each body being separated from all the others.

The administration of monosilane should be applied uniformly over several nine levels in the container.

In the invention, the homogeneous reaction that usually follows pyrolysis on monosilane red-hot silicon rods can be prevented or inhibited. Therefore, the diameter of the growing ultra-pure silicon rods may be uniform and the amount of silicon deposited per unit of time is increased .. As the amount of powdered silicon that could be incorporated into the rods is substantially reduced, the rods produced are of higher quality.

Claims (2)

A process for producing uniformly cross-sectional silicon rods, wherein a silicon-containing compound is subjected to pyrolysis on a plurality of ultra-pure silicon support bodies, the support members being pressurized electrically to heat the bodies to a temperature incandescent, thereby ultra-pure silicon is deposited on the bodies, introducing monosilane into a pyrolysis container surrounding a carrier through multiple-level supply orifices, characterized by thermally shielding the support bodies and increasing the proportion of monosilane added through the upper orifices. as the rod diameter grows at the silicon deposition. Apparatus for use in the method according to claim 1 and consisting essentially of a pyrolysis container, said rod-shaped supporting bodies of ultra-pure silicon and a feeder tube for the silicon-containing compound, wherein the feeder tube has 20 or more feeder mouths in the length of the support bodies. , characterized in that the supply orifices (6) are located at three levels, namely an upper, a middle and a lower, that a thermal insulator (1) is provided in the pyrolysis vessel (3) which shields the supporting bodies (1). that the feed tube (17) extends into a cavity in the interior of the insulator (5), that the supply orifices (6) are formed in end faces of heat insulating plates constituting the insulator (5) and that the supply tube (17) is built into flow regulators (C) and flow dividers (S).